add venv instructions to README.md

[mrariden]

Some users may want to use a python version already present on their machine. The easiest way to do this is with python's native venv.

I created cellpose installation instructions for installing venv for mac/linux/windows.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.26%. Comparing base (cc4e62c) to head (7e0e6d3).
Report is 14 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #892   +/-   ##
=======================================
  Coverage   56.26%   56.26%           
=======================================
  Files          16       16           
  Lines        3784     3784           
=======================================
  Hits         2129     2129           
  Misses       1655     1655           

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